Immersion-plated tin has been used as one of the alternative final finishes for printed wiring board (PWB) because it provides a uniform metallic coating for improved in-circuit-test (ICT) probe life, lubricity for press fit pins, and excellent solderability. Because of the strong affinity between copper and tin, inter-diffusion occurs spontaneously even at room temperature through bulk, grain boundary, and surface diffusion pathways, resulting in the formation of intermetallic compounds at the Sn/Cu interface as well as in the grain boundaries of tin-based coating layers.
Tin surfaces are used in the manufacture of printed circuit boards, IC substrates, semiconductor wafers and related devices as a final finish, serving as a solderable or bondable surface for subsequent assembly steps. Tin is primarily deposited onto copper features of a substrate, particularly contact points. The method of choice for this application is deposition of tin by electroless plating procedures with immersion plating as the most commonly applied method. The immersion plating process of tin or tin alloys onto a copper surface operates according to the following reaction:Sn2+plus 2Cu→Sn plus 2Cu+
Thus divalent tin ions are reduced to tin metal by copper metal and the copper is oxidized into ionic form. In most cases the monovalent copper ions are further oxidized by oxygen in the solution to divalent copper ions. Immersion tin plating is variously referred to as “electroless tin plating”, “displacement tin plating”, or “conversion tin plating”. As can be seen from the reaction, copper metal is dissolved from the surface of the substrate being plated and tin metal is correspondingly plated on the surface.
The tin layer deposited onto the copper features serves as a solderable and bondable surface for reflow and soldering processes as well as wire bonding. Tin layers for said applications typically have a thickness of ≤1 μm. However, a tin layer having a thickness of ≥1 μm or even ≥5 μm may be desirable.